Electrolytic deposition of zinc



Patented Sept. 4, 1945 ELECTROLYTIC DEPOSITION F ZINC Charles G.Harford, Quincy, Mass., asslgnor to Arthur D. Little, Inc., Cambridge,Mass, a corporation of Massachusetts No Drawing. Original applicationJuly 3, 1937, Serial No. 151,854, now Patent No. 2,355,070, dated August8', 1944. Divided and this application July 12, 1944, Serial No. 544,651

6 Claims.

This invention relates to an improved method 01 electrolysis, includingthe electro-deposition of metals, particularly zinc, and to thecomposition of the electrolyte. This application is a division of thecontinuation-impart application Serial No. 509,198, filed November 6,1943, of application Serial No. 21,809, filed May 16, 1935, and adivision of co-pending application Serial No. 151,854, filed July 3,1937-now Patent No. 2,355,070 of August 8, 1944.

In the procedures heretofore employed in the electroplating art, it hasbeen found advantageous for various reasons to prepare an electrolyte inwhich the metal to be deposited is in the form of a complex compound. Asa result of extended experience and experiment, cyanides have been foundespecially suitable and are used in large amounts in electrolytic baths,and many technical practices of electrolysis and electroplating dependupon them. Such practices have been adopted and adhered to, in spite ofthe wellknown intensely poisonous character of cyanides and cyanidesolutions and the danger which is consequently incurred throughout theplants in which such compounds must be stored, handled, operated withand replenished from time to time.

Other procedures have been followed, it is true, but these, in general,entail the use of two or more solutions operating as a batch process.Operating in this way it is necessary to add chemicals to certain partsof the system and to withdraw and discard worthless by-products fromothers. In thus destroying or converting the components of theelectrolyte to some other form, these processes of the prior art involvethe unnecessary and hence wasteful consumption of reagents and ofelectric current or energy, as well.

It is accordingly an object of this invention to provide an improvedmethod of electro-deposition in which satisfactory or improved resultsmay be accomplished and in which compounds other than the cyanides, andwhich are essentially non-poisonous, may be used.

Another object is to provide. a method in which the current efliciencymay be improved. A further object is to obtain a brighter plate.

It is also an object to provide an electrolytic process and anelectrolytic medium wherein the reaction consists in effectingelectrolytic transfer of the metal (anode) to metal (cathode) withoutside reactions or at least without side reactions resulting in thecumulative formation of ancillary iby-products, and in which the onlycumulative product is that of the electrolytically deposited metal onthe cathode, and in which the only required addition is that ofreplenishing such metal in the system, as a metal at the anode. Economyof reagents and of electric current, accordingly, are objects of theinvention. Other objects will appear from the following disclosure.

A salient feature of the present invention is the discovery that zinc issubject to a uniform electrolysis and undergoes a dependable dissolutionand regular deposition of the metal under the influence of an electriccurrent, if the electrolyte is of substantially uniform compositionbetween the anode and the cathode and contains a hydrocarbon polyaminesuch as ethylene diamine. It may be further enhanced if an excess of thehydrocarbon polyamine is present. The hydrocarbon polyamineadded-whether in excess or not-does not appear to be permanentlyaffected by the electrolysis and even after prolonged use of the bath itis found upon analysis to remain undiminished in its total content."

The hydrocarbon polyamine combines with the metal or with the cation ofthe electrolyte and particularly with the metal component of the salt,to form therewith a complex ion which is readily susceptible toionization, electrolytic transfer to the cathode, and discharge of themetal ion and deposition of the metal in solid form thereon. The metalcontent of the electrolyte is replenished by the direct corrosion anddissolution of the anode.

By the expression electrolytic deposition" as used in the presentapplication and in the claims, it is to be understood the electrolysisof an electrolyte, containing one or more metals in solution, andforming a continuous liquid medium between the cathode and a solublemetal anode, which includes the separation and deposition of saiddissolved metal or metals upon the cathode, and simultaneouslydissolving of the metal anode, thereby replenishing and maintaining themetal content of the electrolyte.

The anodes, formed of zinc, are uniformly and regularly dissolved intothe electrolyte without disintegration and the resulting formation ofresidua1 granules or powder. 4

The process is characterized by being applicable to metals which areable to form complex cations containing a hydrocarbon polyamine. Theproof that such complex ions are formed is that the metals in the formof the complex ion Hydrocarbon polyamines contain carbon, ri-

trogen, and hydrogen and no other elements.

They are thus distinguished from ammonia, and

a least two atoms of nitrogen, each of which is joined to a carbon atombut not to a nitrogen atom; they are water-soluble and definitely basic.

Examples of suitable hydrocarbon polyamines are '1 ethylene diamine,propylene diamine, and the polyethylene amines. The latter areconsidered to be condensation productsof ethylene diamine with itself orwith other aliphatic amines; one of the simplest of such products wouldbe:

The general formula of hydrocarbon polyamines suitable for use inconnection with this process is therefore:

where n is 2 or more and Ris hydrogen or the radical -(CH)2n-m R beingthe same as just explained.

The process of the invention, as described in .the above mentionedapplications, may be carried out, for example, with salts of certainmonovalent metals, such as silver, with salts of bivalent metals, suchas copper, cadmium, zinc, and nickel, with salts of trivalent metals,such as iron and chromium, and salts of tetravalent or hexavalentmetals, such as platinum and tungsten, in the presence of the polyamine.In these metallic polyamine complexes, the polyamine and the metal areattached to each other by the secondary valences of the nitrogen and ofthe metallic atoms. Since only secondary valences are involved, theentrance of the polyamine into the cation does not alter the charge orvalence of the cation.

In practice, it is now found that when zinc is made the anode, withrespect to an electrically conductive cathode, and an aqueous fluidelectrolyte is provided, extending from the surface of one to thesurface of the other without interruption, containing a solublehydrocarbon diamine as above defined-the application of an electricalpotential or current between the electrodes will result in thedissolution of the anode and the electrolytic deposition of the metalupon the cathode, without side reactions and without change ofregulation other than the addition of metal as metal to the anode andremoval of the deposited metal on the cathode.

It is found that a water solution of the amine alone is sufficient forsuch operation-the metal from the anode dissolving and forming complexcations in the electrolyte and undergoing electrolytic depositionwithout the addition of any salt of the metal and without any indicationthat salt formation of the metal occurs. On the other hand, a salt ofthe metal or metals involved may be present to advantage, especiallywhere it is economical to employ.

In either case, however, it is essential that the fluid medium of theelectrolyte between the anode and cathode be unobstructed, continuous,and preferably uniform, as by agitation.

It will be observed that the electrolytic solution is contained in asingle vessel, which is preferably lined with rubber, and forms acontinuous me-- dium between the anode or anodes (which are connected tothe positive bus bars, and are composed of the metal or metals to bedeposited) and the cathode or cathodes, which are connected to thenegative bus bar, and upon which the metal is to be deposited. Theuniformity of the electrolytic solution during operation may be promotedby agitating the electrolyte with any suitable means (not shown) but issubstantially automatically preserved throughout the reaction which isaccurately'representemin its net effective "result, by the equationMetal (anode)-metal (cathode) for there is no cumulative consumption orconversion of the other constituents of the electrolyte into by-productsand hence no necessity for replacement or reconversion, even upon longand continuous operation.

It is found that hydrocarbon polyamines react with ionized salts ofzinc, in aqueous solution, to form various orders of compounds. Thenumber of compounds so formed varies, in accordance with the number ofsecondary valences (as set forth in theories of Werner complexes), butneed not be gone into here in any great detail as the invention will beclear from the disclosure herein.

It is generally preferable to use at least 2 mols of hydrocarbonpolyamine to one mol of the salt of zinc, and best results range betweenabout 2 and 6 mols of the polyamine to one of the salt. More polyaminemay be used, but there is no particular advantage in so doing.

Typical and representative examples of the practical application of thisinvention will now be described in its relation to electroplating, withrespect to zinc and to various salts of the metal.

In the example, the ethylene diamine is given on the anhydrous basis,although it is customarily available in the form of a 40% or 60%solution in water.

In carrying out the process of this invention, the temperature of thebath is preferably at, or somewhat above, room temperature, generallybetween 25" C. and 40 C.

The electrolyte may be contained in any suitable resistant vessel whichis preferably a nonconductor of the electric current, such asearthenware or rubber. The anode (or anodes) may conveniently consist ofzinc strips which are connected to a source of electric current and aresuspended in the solution, preferably 50 as to be completely submerged,as by bus bars, having a chemically resistant or protected conductor.The cathodes were connected to a suitable source of electric current toprovide a current density of 10 to 40 amperes per square foot of cathodearea and were found to give satisfactory electrolytic depositions undercontinuous operation for two to eight hours, without deterioration ofthe electrolyte or deterioration in the rate and quality of metaldeposition. Moreover at the end of a prolonged period of such operations(which was merely discontinued, and not on account of any failure ordecline in operation) the amount of ethylene diamine remaining in theelectrolyte was substantially undiminished.

While ethylene diamine is set forth in the example as the hydrocarbonpolyamine used, it may be replaced by other hydrocarbon polyamines.

The plating of zinc, using zinc anodes and a bath containing a zincsalt, may be carried on with a bath having the following composition:

ZnSO4.7H2O .-grams Ethylene diamine do.. Sulfuric acid To give pH of 10Water To make 1000 c. c.

The plating is carried out by the procedure already described, using atemperature of about 25 0., a metallic zinc anode, and a current densityof to 20 amperes per square foot of cathode surface. The resulting plateof zinc is light, frosty, and metallic in appearance, and has excellentadhesion. Steel, copper or other suitable metal is used as the cathode.

The ratio of ethylene-diamine to salt in the above example is about 6to 1. The working range of this ratio is between 2 and about 10.

The pH value, for best results, should be between 8 and 12. Outside thisrange a spongy, unsatisfactory deposit is obtained.

It is also possible to plate alloys by the present process. For example,brass may be successfully plated by using brass anodes and a bath ofcopper and zinc salts.

As described in the above mentioned application, it is possible to carryout the process of this invention without first dissolving a salt of themetal to be plated in the bath. In such cases, the metal of the anodedissolves to furnish sufllcient metallic ions in the bath, and'platingproceeds. Such procedure has no particular advantage when plating withmost ordinary metals, but when the rarer metals are used, such astungsten and platinum, and metals of the platinum group, it is generallyinconvenient and expensive to use their salts in the bath. and adequateresults can be obtainedas' far as such metals are concerned when ananode of the metal to be plated is placed in a bath containing ahydrocarbon polyamine and the plating carried out as described above.For instance, a tungsten anode is immersed in a bath containing25%ethylene diamine in water.

Platinum and other metals of the platinum group may be plated in thesameway. Alternatively, if desired, salts of these metals may be addedto the plating baths and the procedures of the precedingexamples may beused.

The cathode, in operating with the solutions described herein, may beubstantially any clean, smooth, electrically conductive metallicsurface. The operation of the reaction is smooth and continuous, forms abright, uniform deposit of metal on the cathode, and may be prolongedfor substantially any desired period of time without care or control,other than the supply of an electric current, and replenishment of themetal of the anodes, and removal and replacement of the plated cathode,from time to time, if and when desired.

If there is any tendency for basic salts of the electrolyte to separate.as for example, at the cathode (with solutions containing a lower ratioof hydrocarbon polyamine to metallic salt than 2:1), which mightinterfere with the continuity of the deposition or of the plated surfacecoating, this may be overcome by the addition of sulfuric acid (in caseof sulfate salts or corresponding acid in case of other salts) or of thepolyamine, or both. Agitation of the bath or of the cathode is likewisehelpful.

I claim:

1. An aqueous electrolyte for the electrolytic deposition of zinc,characterized by consisting essentially ofa' soluble salt of zinc and analkyl hydrocarbon polyamine, and manifesting a pH value betweenapproximately 8 and 12.

2. An aqueous electrolyte for the electrolytic deposition of zinc,characterized by consisting essentially of a soluble salt of zinc andethylene diamine and manifesting a pH value between approximately 8 and12.

3. A process of electrolysis that comprises electro-depositing zinc froman undivided cell containing an aqueous electrolyte consistingessentinally of a soluble salt of zinc and an alkyl hydrocarbonpolyamine and manifesting a pH value 6. A process of electrolysis thatcomprises electro-depositing zinc from an .undivided cell containing anaqueous electrolyte consisting essentially of a soluble salt of zinc andan alkyl hydrocarbon diamine, and manifesting a pH value betweenapproximately 8 and 12.

' CHARLES G. HARFORD.

